Two of the most popular consumer electronic devices are the cordless telephone and the cellular telephone. Cordless telephones consisting of a handset and a base that communicate via a radio link are used pervasively in the home and office. The cordless phone's popularity stems chiefly from the user's ability to roam freely while talking on the cordless phone. Cellular phones extend this freedom beyond the home and office by allowing a cellular phone user to communicate with a base station in a predefined cellular area. As the user roams from cell to cell, the cellular network automatically transfers the user from base station to base station.
As in the case of battery-powered laptop computers, it is a key design objective to minimize the power consumption of cordless and cellular telephones, thereby extending the time between recharges of the handset batteries. Most users of portable radio telephones (whether cellular or cordless) carry the phones in "standby" mode the majority of the time. In standby mode, the RF receiver is "ON", but the transmitter is "OFF". This saves power and extends battery life. However, even in "active" mode, where the user is engaged in a phone call, it is possible to save power under certain operating conditions.
One of the operating parameter signals used in cellular telephones is a received signal strength indicator (RSSI), which provides an accurate estimate of the signal strength received from a base station. RSSI is used for numerous purposes, including prompting the current base station to transfer monitoring of the mobile cellular to a new base station when the current base station no longer provides a satisfactory signal. The base station may also use RSSI data received from its associated mobile cellular phones to increase or decrease its transmitting power. In the mobile cellular phone, RSSI may be used to adjust receiver gain in those phones which employ automatic gain control (AGC) amplifiers. In a like manner, cordless telephones may also derive a RSSI signal in the handset, which signal is then used to adjust the transmission power of either the handset or the base, or both.
When the received "wanted" signal is very strong, there is minimal risk of interference from other large signals, and the ever present background noise is much smaller than the wanted signal. Under these conditions, there is no need for the high gain and anti-interference performance associated with wanted signals that are relatively weak and noisy. It is therefore possible to reduce bias current in some of the receiver components, thereby conserving power.
For example, switchable gain low-noise amplifiers (LNA) have been implemented in RF receiver front-ends to minimize total power supply current when the RF receiver is operating in half duplex mode. If the RF receiver switches to full duplex (i.e., the transmitter operates simultaneously with the receiver), the LNA current is switched to full power to increase the compression point and avoid interference from the high-power transmit signal coexisting on the antenna at the same time as the minimum strength received signal.
However, the front ends of conventional RF receivers also contain RF mixers that operate using substantially constant bias current, independent of the strength of the received wanted signal. The power supply current in an RF mixer may typically be 6-10 mA (or more). Yet when the received signal is strong, such as when a mobile cellular phone is near the base station or a cordless handset is near the base, only 1-2 mA of current may be required for proper operation of the RF mixer. Thus, under certain operating conditions, the amount of current in the RF mixer can be reduced, thereby reducing power consumption. The current may be reduced less is needed to establish a level of intermodulation distortion which protects a strong "wanted" signal from interfering signals.
Therefore, there is a need in the art for improved RF receivers employing RF mixers that consume less power under strong received signal conditions. There is a still further need for improved RF receivers that employ switchable gain RF mixers that may be switched between a high-power mode and a low-power mode depending on the value of a received signal strength indicator (RSSI).